Models of ectopic activity, both reentrant and parasystolic, have been developed in three systems: (1) a sucrose gap preparation of Purkinje tissue (false tendons) or ventricular muscle trabeculae; (2) preparations of the right bundle branch and ventricular septum in which local deplorization can be induced by extracellular current injection; and (3) a mathematical model programmed to stimulate the tissue models. Frequency-dependent patterns of ectopic activity generated in these models will be studied in detail to demonstrate the sometimes subtle distinctions between "reentrant" and "pacemaker"-generated arrhythmias. Pacemaker activity generated by digitalis, or modified by digitalis in these models, as well as the responses to anti-arrhythmic drugs will also be studied. The mathematical model of modulated impulse generation and conduction, and of pacemaker interactions, will be further developed to predict patterns of ectopic arrhythmias and to assist in the formulation of diagnostic criteria.